So we can summarize by saying that the photosynthetic plantstrap solar energy to form ATP and NADPH (Light Phase) and thenuse these as the energy source to make carbohydrates and otherbiomolecules from carbon dioxide and water (Dark Phase),simultaneously releasing oxygen in to the atmosphere. Thechemoheterotrophic animals reverse this process by using theoxygen to degrade the energy-rich organic products ofphotosynthesis to CO2 and water in order to generate ATP fortheir own synthesis of biomolecules.
Photosynthesis converts these energy- depleted compounds (ADPand NADP+) back to the high energy forms (ATP and NADPH) and theenergy thus produced in this chemical form is utilized to drivethe chemical reactions necessary for synthesis of sugars andother carbon containing compounds (e.g., proteins, fats). Theproduction of high energy ATP and NADPH in plants occurs in whatis known as Light Phase Reactions (Z Scheme) (requiressunlight). The energy releasing reactions which converts themback to energy-depleted ADP and NADP is known as Dark PhaseReactions (Calvin Cycle) (does not require light) in whichthe synthesis of glucose and other carbohydrates occurs.
For each electron flowing from water to NADP+ (a net change in1.14 volts), two quanta of light are absorbed, one by eachPhotosystem. Each molecule of oxygen released involves the flowof four electrons from two water molecules to two NADP+s andrequires four quanta of sunlight absorbed by each Photosystem toprovide the energy to do this. These are the "Light PhaseReactions" of photosynthesis, which produce two high energychemical products, namely NADPH and ATP.
The Z Scheme diagram shows the pathway of an electron fromwater (lower right) to NADP+ (upper left). It also shows theenergy relationships which are measured as voltage potentialshown on the scaleon the right. To raise the energy of theelectrons derived from water (+0.82 volts) to the level necessaryto reduce NADP+ to NADPH (-0.32 volts), each electron must beboosted twice (vertical red arrows) by light energy absorbed inPhotosystems I and II. After each boosting , the energizedelectrons flow "downhill" (diagonal black lines) and inthe process transfer some of their energy to a series ofreactions which ultimately adds a phosporus to ADP to producehigh energy ATP and reduces NADP+ to NADPH. There is analternative shunt whereby the electron flow turns back tocytochrome b563 (green line)and this is called and it occurs when there is no need for NADPH, so onlyATP is produced.
They are also the sites for nitrogen fixation, the reactions of which are interconnected with the processes of photosynthesis and photorespiration, and play a protective role in plant cells.
When the chlorophyll molecule is excited by light, the energylevel of an electron in its structure is "boosted to ahigher energy level and this "excited" chlorophyll (nowis called an ) moves rapidly the the reactioncenter of the Photosystem I where it transfers its extra energyto an electron which is then expelled from the reaction centerand is accepted by the first member of a chain of electroncarriers and ultimately reaches NADP+, reducing it to NADPH. Thereaction center has lost an electron and this "electronhole" is filled by by stripping electrons from water whichleaves hydrogen ion (H+) and molecular oxygen (O2). The pathwayof electrons from water to NADP+ has "Z" shape whendiagramed and is refered to as the Z Scheme.
In PS I, the electrons are again excited by harnessing the energy from photons, and the reduction of NADP to NADPH2 is achieved by utilizing electrons and protons.
Stroma is the site for the dark or light-independent reactions of photosynthesis.
Chloroplasts have many shapes in different species but aregenerally fusiform shaped (and much larger than mitochondria) andhave many flattened membrane-surrounded vesicles called thylakoidswhich are arranged in stacks called grana. Thesethylakoid membranes contain all of the photosynthetic pigments ofthe chloroplast and all of the enzymes required for Light Phasereactions. The fluid in the stroma surrounding the thylakoidvesicles contains most of the enzymes for Dark phase reactions.
Plants absorb water through their roots, and carbon dioxide through their leaves. Some glucose is used for respiration, while some is converted into insoluble for storage. The stored starch can later be turned back into glucose and used in respiration. Oxygen is released as a by-product of photosynthesis.
Plant photosynthesis, both the Light Phase and Dark phasereactions, takes place in chloroplasts, which may be regarded asthe "power plants" of the green leaf cells. At night,when there is no sunlight energy, ATP continues to be generatedfor the plant's needs by respiration, i.e., oxidation of(photosynthetically produced) carbohydrate in mitochondria(similar to animals).
During this reaction, and water are converted into glucose and . The reaction requires , which is absorbed by a green substance called chlorophyll.
While oxygen is necessary for the process of respiration, glucose plays a crucial role in the diet; and that explains why the photosynthesis is important for all the lifeforms on the planet - including humans.
In a broad chemical sense, the opposite of photosynthesis isrespiration. Most of life on this planet (all except in the deepsea vents) depends on the reciprocal photosynthesis-drivenproduction of carbon containing compounds by a series of reducing(adding electrons) chemical reactions carried out by plants andthen the opposite process of oxidative (removing electrons)chemical reactions by animals (and plants, which are capable ofboth photosynthesis and respiration) in which these carboncompounds are broken down to carbon dioxide and water.
This process is called photosynthesis. Temperature, carbon dioxide concentration and light intensity are factors that can limit the rate of photosynthesis.